Electronic screening for image reproduction is well established in the art. According to a well known technique described in U.S. Pat. No. 4,456,924 of the present assignee, for each screened dot, a multiplicity of coordinates of a laser plotter are translated into screen-cell coordinates. A corresponding cell memory is preloaded with threshold values, defining a cell memory matrix. Input digitized scanned density values of an image, such as a color separation, are compared with the threshold values, cell by cell. The results provide an on/off control input for a laser plotter.
U.S. Pat. No. 4,825,298 to Ikuta and Murai describes a technique for generating a screened reproduction of an image in which the density distribution of a given screen dot is expressed in three dimensions, wherein the area of the screen dot is expressed along X and Y axes and the density is expressed along a Z axis perpendicular thereto. A film coordinate generator generates film coordinates (u,v), corresponding to the position of an exposure beam on a recording film which position is detected by encoders. The film coordinates are in turn supplied to a screen coordinate generator to be converted into virtual screen coordinates (x,y). A beam control signal generator receives the coordinates (x,y) and an image signal corresponding to the position of the exposure beam to output a beam control signal indicating lighting of the exposure beam when one of the coordinates (x,y) is between upper and lower limit values, corresponding to the same, which are previously determined for each combination of the other of the coordinates (x,y) and the density value of the image signal. U.K. Published Patent Application 2,157,119A to Ikuta describes apparatus which operates similarly to the technique of U.S. Pat. No. 4,456,924 but does not employ a matrix memory. Instead, the threshold function is calculated on the fly in real time or near real time. This apparatus is limited to relatively simple dot configurations.
In U.S. Pat. No. 4,918,622 there is described an electronic graphic arts screener in which a three-dimensional memory array is employed for screen dot generation.
U.S. Pat. No. 4,447,833 describes a method for producing a halftone plate in which control of the luminous energy is performed by the sum, the difference, the product or the quotient of a reference value corresponding to the picture signal and a position signal value varied depending on an exposing position in the halftone dot area to be exposed by an exposure light beam. The patent proposes techniques which do not provide accurate control of incremental halftone dot growth. U.S. Pat. No. 4,985,779 describes a method and apparatus for generating halftone images employing a beam of variable intensity. The apparatus and method employs a lookup table that contains, at addressable locations, output data defining the desired intensity of the radiation beam, as a function of input information, including screen memory coordinates and picture intensity data.